Thin-film solar cells being photoelectric conversion elements may, for example, be of an amorphous silicon (a-Si) type or a polycrystalline silicon type. In such thin-film silicon type solar cells, a transparent conductive oxide film is used as an incident light side electrode. Such a transparent conductive oxide film is required to have a low resistance and high transparency and to have a high light scattering performance in order to increase the photoelectric conversion efficiency. JP-B-7-105166 discloses a fluorine-doped SnO2 film which contains fluorine in an amount of from 0.01 to 4 mol % based on SnO2 and which has a conductive electron density of from 5×1019 to 4×1020 cm−3, and the film has a low absorbance, is highly transparent and further has high durability against an active hydrogen species.
JP-B-6-12840 discloses a transparent conductive film which has a surface roughness (texture) structure and has an effect to scatter incident light within a photoelectric conversion unit, whereby as compared with a transparent conductive film having a small surface roughness, the photoelectric conversion efficiency of an amorphous silicon solar cell can be made high.
On the other hand, in the case of a thin-film crystalline silicon solar cell such as a thin film polycrystalline silicon or a thin-film microcrystalline silicon which has been actively studied in recent years, the cell sensitivity in a long wavelength region is high as compared with an amorphous silicon solar cell. This indicates that as compared with an amorphous silicon type, a light scattering property and a high transparency in a longer wavelength region, are required for the transparent conductive film. In order to increase light scattering at a long wavelength, it is effective to further increase the surface roughness structure of the transparent conductive film. For example, if the film thickness is made thick, the crystal grain size will be increased, whereby the surface roughness can be increased. However, a transparent conductive film such as a fluorine-doped SnO3 film has light absorption in a long wavelength region by free electrons, whereby if the film is made thick, the light absorption increases, whereby the optical transmittance decreases. Consequently, even if light scattering on a long wavelength side is increased by increasing the surface roughness, light absorption of a long wavelength will also increase, whereby as a whole, the photoelectric conversion efficiency of the solar cell will not increase, and it has been difficult to increase the efficiency of the photoelectric conversion by means of a transparent conductive film having a high spectral haze value (which may hereinafter be referred to simply as “a haze”).
Other than the above, a technique to increase the light scattering effect by controlling the surface roughness of a transparent conductive film in contact with the photoelectric conversion layer, has heretofore been well known and is disclosed in e.g. JP-A-3-125481, JP-A-2000-252500, JP-A-61-288314, JP-A-61-288473, JP-A-61-288314 or JP-A-2000-232234.
In JP-A-3-125481 among them, a transparent electrode substrate is disclosed which is characterized by a structure wherein a first layer having a large average particle size and a second layer having a small average particle size are laminated. This is designed to refract and scatter light with a long wavelength by the first layer having a large average particle size and light with a short wavelength by the second layer having a small average particle size, in order to let more light be absorbed by the photoelectric conversion layer. However, with the electrode structure disclosed in Examples, both the first and second layers are transparent conductive films, whereby absorption by free electrons can not be avoided. Namely, incident light will pass through the first layer film of at least 1.0 μm over the entire region of the substrate surface and will further pass through the second layer film of at least 0.2 μm, whereby as a whole, absorption by films of at least 1.2 μm will take place. Accordingly, attenuation of light before reaching the photoelectric conversion layer can not be avoided. Thus, it has been found that with the construction of the substrate as disclosed in JP-A-3-125481, no significant improvement-can be obtained in the photoelectric conversion efficiency.
Further, JP-A-2000-252500 also discloses a transparent electrode substrate for a silicon thin-film type photoelectric conversion device, wherein a first transparent conductive film having a large difference in the surface roughness, is formed on a glass substrate, and a second transparent conductive film having a small difference in the surface roughness, is formed thereon. It is stated that by reducing the difference in roughness of the second transparent conductive film to make the surface smooth, spike-like protrusions can be eliminated, whereby short circuiting of junctions in the photoelectric conversion unit can be reduced, and thus fluctuation of the performance of the photoelectric conversion device can be reduced. However, it has been found that also this transparent electrode substrate has a drawback that, like the above-mentioned problem, as light will pass through absorptive two layers of transparent conductive films (continuous films), the amount of incident light to the photoelectric conversion layer will be reduced by an amount absorbed by the conductive films, whereby the photoelectric conversion efficiency will not be improved.
Further, JP-A-61-288314 and JP-A-61-288473 disclose that with a transparent electrode film represented by indium/tin oxide or SnO2 formed by a conventional electron beam vapor deposition method, a vacuum vapor deposition method, a sputtering method, a CVD method or a spray method, the difference in the surface roughness is from about 20 to 100 nm, and the distance between protrusions is from about 50 to 200 nm, whereby the light scattering effect at the interface with the photoelectric conversion layer is inadequate. Whereas, it is disclosed possible to increase the light scattering effect at the interface and to increase the photoelectric conversion efficiency by carrying out chemical etching treatment of the transparent electrode film surface to form a roughened surface having a difference in roughness of from about 100 to 500 nm and a distance between protrusions of from about 200 to 1000 nm. However, this system requires to carry out chemical etching treatment after forming the transparent electrode films and to sufficiently clean and dry the substrate in order to remove the etching solution and then to form the photoelectric conversion layer, whereby the process tends to be cumbersome, and there is a problem that the productivity is low.
Further, JP-A-2000-232234 discloses that a photoelectric conversion device having a transparent electrode wherein the difference in surface roughness is from 10 to 100 nm and the pitch of surface roughness is larger than the difference in the surface roughness and not larger than 25 times thereof, will have the photoelectric conversion characteristics improved by a light-trapping effect, without bringing about a decrease of an open circuit voltage or a decrease of the production yield. However, the means to realize the surface roughness in this process is chemical etching like in the above-mentioned cases, whereby the process tends to be cumbersome, and there will be a problem in mass production.
The present invention has been made to solve such problems of the prior art. It is an object of the present invention to provide a substrate with a transparent conductive oxide film (especially a substrate with a transparent conductive oxide film useful as a substrate for a thin film silicon type solar cell) which has a low resistance, a high transparency and a characteristic of having a good light scattering performance over a full wavelength region (from 300 nm to 3 μm) of solar ray and which is excellent in mass productivity, a process for its production, and a photoelectric conversion element (especially a solar cell) employing such a substrate.